1. Field of the Invention
This invention relates to a process for preparing oil-in-water emulsions and cosmetics comprising such emulsions.
2. Description of the Prior Art
Emulsifying techniques are known for systems other than the oil-water system. For example, Petersen and Hamill conducted experiments on the emulsification of olive oil as a non-polar phase and glycerin, propylene glycol, or polyethylene glycol as a polar phase using various surface active agents. Furthermore, Petersen, Hamill and MacMahon emulsified glycerin and olive oil using a combination of a nonionic surface active agent and another surface active agent. The prior art, however, failed to provide emulsions having good stability. No consideration has ever been given to methods for preparing stable oil-in-water emulsions by adding water to an emulsion of oil in a water-soluble solvent.
Du Pont Aerosol Guide Book, 1969, contains some information on an emulsifying process in which a water-soluble solvent (propylene glycol) is added to a mixture of an oil and Freon gas. This report, however, does not relate to processes for preparing oil-in-water emulsions as in the present invention.
In the preparation of oil-in-water emulsions according to conventional techniques, the composition and the amount of a liquid phase are first determined, and then, the type and amount of emulsifier suitable therefor and the emulsifying method are chosen.
In selecting the emulsifier, the empirically established HLB values developed by Griffin et al have generally been utilized. This concept, however, essentially relates to the solubility of surface active agents in an oil-water system; generally, a surface active agent is dissolved in an oil phase or in an aqueous phase after its HLB has been adjusted to 7-18 in the case of an oil-in-water emulsion or to 3-7 in the case of a water-in-oil emulsion.
A special method is sometimes used in food engineering which comprises adding small amounts of an oil phase and an aqueous phase alternately to a surface active agent phase to form an emulsion.
In order to obtain a stable emulsion using a nonionic surface active agent, it is necessary to use a combination of a hydrophilic non-ionic surface active agent and an oleophilic nonionic surface active agent, and to orientate the surfactants at the interface of the oil and aqueous phases by increasing the area of the interface using a strong stirring means, e.g., a homogenizer. This requirement, however, is not the case when more than one kind of surfactant is used and large amounts of surfactants are used.
However, when the hydrophilicity of the hydrophilic nonionic surfactant is too high (e.g., an HLB of at least about 20), its surface activating ability is reduced, which makes it difficult to obtain a stable emulsion even when an oleophilic nonionic surfactant is used in combination therewith.
Generally, the stability of emulsions prepared by conventional emulsifying methods is not good at high temperatures because the non-ionic surfactants used have a relatively low cloud point. This problem is not generally encountered when ionic surfactants capable of providing electric charges on the interfacial membrane between the two phases are used. In such a case, emulsified oil droplets repel each another upon approaching because of the charging of the interface, and the coalescing or associating phenomenon seen in the case of using a nonionic surfactant scarcely occurs. Hence, an emulsion of good quality at high temperatures can be obtained. In order to obtain stable emulsions using nonionic surface active agents as are commercially available, it has been essential to use the same in combination with relatively large proportions of an ionic surface active agent.